This invention is concerned with devices, such as mirrors and windows, having controllable transmittance and reflectivity.
Sunlight transmitted through windows in buildings and transportation vehicles can generate heat (via the greenhouse effect) that creates an uncomfortable environment and increases air conditioning requirements and costs. Current approaches to providing "smart windows" with adjustable transmission for use in various sunlight conditions involve the use of light absorbing materials. These approaches are only partially effective, since the window itself is heated and because these devices, such as electrochromic devices, are relatively expensive and exhibit limited durability and cycle life. Certain liquid crystal-based window systems switch between transmissive and opaque/scattering states, but these systems require substantial voltages to maintain the transparent state. There is an important need for an inexpensive, durable low voltage smart window that can be adjusted between transmissive and reflective states. Reflecting the light, rather than absorbing it, is the most efficient means for avoiding inside heating.
In prior art attempts to exploit reversible electrodeposition of a metal for light modulation, the deposits obtained on transparent substrates presented a rough and black, gray, or sometimes colored appearance (typical of finely-divided metals) and exhibited poor reflectivity and high light absorbance, especially when thick. This was true in the work of Udaka, for example, even when the transparent conductor electrode surface had been metallized (Udaka, et al., published European Patent Application No. 0712025, Application No. 95117797.1). Such deposits have been investigated for display applications involving reflectance from the background, with white pigments often being added to improve contrast. Warszawski (U.S. Pat. No. 5,056,899), which is concerned with displays, teaches that reversible metal electrodeposition is most appropriate for display applications, since significant disadvantages for transmission devices were given (e.g., the possibility of blocking radiation via metal deposition at the counter electrode). Such teachings imply that the application of reversible metal deposition to smart windows must involve light absorption by the finely divided electrodeposited metal, which would result in heating of the device itself and thus the space inside. The prior art literature also teaches that, for transmission-type devices, reversible metal electrodeposition requires the use of an auxiliary counter electrode reaction; otherwise, metal would plate on the counter electrode as the deposit was de-plated from the working electrode.
Electrolytes described in the prior art literature contain auxiliary redox species (e.g., bromide, iodide, or chloride) that are oxidized (e.g., to bromine, iodine, or chlorine) at the counter electrode during metal deposition, introducing chemistry-related instabilities during long term operation and greatly reducing the memory effect by causing dissolution of the metal deposit on open circuit, e.g., 2Ag.sup.0 +Br.sub.2.fwdarw.2AgBr. In most cases, this auxiliary redox process hinders metal deposition at the counter electrode during erasure of the light modulating deposit, introducing a threshold voltage that is desirable for display applications. This auxiliary redox process represents a significant side reaction even when metal plating/deplating occurs at the counter electrode and a threshold voltage is not observed. See, e.g., Warszawski, Columns 3-4 (when copper or nickel were present in the counter electrode paste) and Duchene, et al., Electrolytic Display, IEEE Transactions on Electron Devices, Volume ED-26, Number 8, Pages 1243-1245 (August 1979); French Patent No. 2,504,290 (Oct. 22, 1982). High switching voltages of at least 1 V were used for all the electrodeposition devices which have been found in the patent and literature prior art.
Warszawski teaches that the use of a grid counter electrode would give a less uniform deposit since deposition on the transparent working electrode is highly localized in the vicinity of the counter electrode grid lines (a consequence of the very thin film of gel electrolyte used). Warszawski also teaches the use of an aqueous gel electrolyte to minimize sensitivity to atmospheric contaminants and to avoid the necessity of having a leak tight seal. Such electrolytes, however, have much more limited temperature and voltage operating ranges compared with organic-based electrolytes with high boiling solvents.
Prior art literature teaches that the memory effect is temporary. This is a consequence of the occurrence of a counter electrode reaction other than metal plating/deplating. The energetic oxidation products generated at the counter electrode can cause dissolution of the metal deposit on the working electrode either chemically on open circuit (slow) or electrochemically during short circuit (fast).
None of the reversible electrodeposition devices known in the prior art have exhibited high-reflectivity mirror deposits as needed for applications requiring adjustable reflectivity. Such deposits have recently been obtained via the use of a surface modification layer by the inventors of the present invention, as disclosed in co-pending patent application Ser. No. 09/333,385, filed Jun. 15, 1999, which is assigned to the same assignee as the present case. The reversible electrochemical mirror (REM) deposits obtained in this case, however, exhibited a high degree of specular reflectivity, which is not always desirable. For example, light from the sun or automobile headlights specularly reflected from REM windows in buildings or automobiles might injure the eyes of bystanders, or might create a safety hazard by temporarily blinding people, especially those driving automobiles. In fact, many governments, particularly in Europe, expressly prohibit highly reflective windows and have imposed maximum reflectivity standards that must be complied with. On the other hand, the heat and light-rejecting characteristics afforded by REM smart windows are still very beneficial. Thus, a need exists for a REM smart window which can be adjusted between transmissive and reflective states, while avoiding the blinding effect that can result from specularly reflecting surfaces, i.e., those that reflect in a highly directional way.